Pet Peeve - A marine said " I want a 556 AR, Not a 223"

I apologize if this has been covered, but…

Recently in a tattoo place, I heard a Marine carrying on loudly about how when he bought an AR15, he wanted an AR556, NOT AN AR223. Clearly he was simply ignorant on the matter, so let’s clarify for any newbs here.

The dimensions, inside and out, of all AR15’s are exactly the same.

The difference between 556 ammo and 223 ammo that they can all fire is in the thickness of the brass at the base, close to the rim where the cartridges are extracted.
556 is a NATO round and used for full auto fire, so that part of the brass is simply thicker to avoid it ripping during hot rapid extraction in full auto fire.
And that my friends, is the ONLY difference between 556 NATO and 223REM ammo. The actual rifles have zero differences on inside dimensions else theyd not all fire both 556NATO and 223REM ammo.

Be aware. Dont perpetuate a myth.


I don’t think it has! That is so good topic! Thanks for your share, sir!

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Hmmmm, I’d have to disagree with you a little here @Smiddy.

I agree with you in that the dimensions of the ammo is the same but the loads are different with 5.56 being higher pressured and AR15’s chambered for 5.56x45 are a little larger and can handle those pressures vs a .223 chamber that may/may not. So while maybe not enough to be a big deal in most circumstances…there is a difference and there is a risk if you’re shooting 5.56 out of a .223 rifle.

Excerpt from the article (because it is long with lots of research and data)
.223 Remington – Because it’s the only SAAMI standardized cartridge, it’s the only one you’re likely to find used in rifles produced by major manufacturers. I would prefer any of the other three to “regular .223.” However, you will not be at a huge disadvantage as long as you buy mostly .223 ammo. As stated above, you are not likely to encounter major problems with limited amounts of 5.56 in a .223 rifle fired out of necessity or in an emergency. Doing so at a high volume for the long term is probably not a good idea. In terms of a carbine-style AR-15, I see absolutely no reason to purchase one with a .223 Remington chamber.

Agreed, good topic!


Pardon me for my lack of understanding; it seems, to the NRA, and more importantly as a SAAMI specification there is a difference which can be over the long term even dangerous to the owner/user.

5.56 NATO vs. .223 Rem.: What’s the Difference?

Please share with me why the reference for design and proper loading of cartridges - at least, as far as I know - would make such a statement if they didn’t have good reason?


Shooting 5.56 out of a .223 is smarter than shooting .40 s&w through a 10mm.

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Probably…but I don’t know that I’ve seen anyone proposing that (if I missed that thread I apologize), and I would disagree with doing that as well.

Just because you can do a thing…does not necessarily mean you should do…that thing…(Federation President in speech at Khitomer; Star Trek VI :slight_smile: )


Is that about as good as 380 kurz in a 9 x19?
I’m just trying to get a good idea of worst “almost as good as…” through ?best? almost as bad?
oh and toss on 38spl in a 357Mag as I’m really wondering if it’s in the hundreds or thousands of rounds 38 before you get too much bore scarring in the cylinder or lead shaving at the forcing cone…

/edit an hour later: One I could see, that would be much more dangerous than the 40 through a 10… would be 357SIG feeding a 10mm… I’d think seriously bad mojo, wouldn’t you? I have two handguns which are capable of being 9x19, 357SIG or 40S&W. Speaking of Obsessive Compulsive Double-checks!


It’s always been my understanding that it is safe to fire a 223 in a 556 rifle, but not the other way around.

Here’s a link to the explanation by Shooting Illustrated “5.56 NATO vs. .223 Rem.: What’s the Difference?”

Although I can’t vouch for him, I sometimes watch this guy’s YT channel. He talks about it in this video:

A side note… although not a .556, when I had my Mini 30 (7.62x39) accurized, they originally cut the chamber for military 7.62x39, and the chamber was so tight, that civilian 7.62x39 wouldn’t feed in it. I had to send it back to accept both as availability of Russian surplus ammo was drying up.


K, I disagree with many of the of the replies.

Heres the math:
5.56mm / 25.4
= 0.2188976378
So, is 5.56 EXACTLY .223 in? No. However, order a receiver for an AR, measure inside dia. of any 2 chambers, and they’ll be identical, says my cousin, armored, gun store owning friend who builds my stuff, and whose the best machinist I know. He builds and repairs enough rifles that I’ve zero reason to disbelieve his word. The guy turns his own 1911 firing pins for crying out loud…

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and any gun steel milled into a barrel qualified to .223 Remington will be (due to the phenomenal qualities of modern stock steel) satisfactory to withstand the difference in loading pressures exerted by a round loaded to 5.56 NATO? am I getting this right?

Look I have nothing but props for your Brother who sounds like he’s a heck of a Smith and has got a rockin CNC shop. But, people don’t say what they say to make up FAKE Truth… unless this is a political discussion and I’ll want none of that.

Let me put it this way, the difference in steel quality is the difference between a submarine valve withstanding a dive depth of 113 feet or 128. If my specification is I ordered that valve made to the 5.56 standard I’d expect my ship to keep water out of the people tank down to 128 feet. If that valve was made to the 223 specs, and I ordered the ship to dive to 128 feet… the valve would only survive to 113 feet of depth; catastrophically fail (maybe not first dive or tenth but it will fail sooner or later), and the little brown fishies would be inside the boat -not swimming around it!

Do you really want to tell people “'s gud nuf ferme”? I’m sorry Sir we live in different worlds and I pray “s gud nuf” keeps your boat afloat in your life. Please don’t be anywhere near me and mine.


@CHRIS4 I smell Amine and Diesel fumes.

OOD: Dive, Submerge the boat.
DO: Helm, Full Dive on the fairwater planes
DO: Stern 15* down bubble
DO: Chief of the Watch Vent forward MBT and sound the diving alarm.
COW on 1MC: DIVE! DIVE!! Ahhhooooghhhaa,!! Ahhhooooghhhaa!! Dive! Dive!!
Corpsman: I’m going to bed, wake me up for Mid Rats.

I agree with your metallurgy assessment. 223 in 5.56 not the other way around.




Ok,perhaps some further research on my part is needed. I’m not a gunsmith. However, I am very qualified with ballistic systems courtesy of Uncle Sam’s training, and completely get what you’re saying about pressures.
Bohrs Law - PV= nRT applies to a round itself, and that round being discharged in a chamber. So, I’ll check my Lymans manual first chance for the data on 223 and compare to 556 for a starting point.

Meanwhile, you metallurgy fellas do me a favor? Check manufacturers and ratings for AR uppers and pressures, and find me one instance where an upper is specified as different for 223 than for 556? I dont build these things, I have professionals do it.

Thx. I’d like to explore this further and be sure I was either incorrect or not from the beginning. If I was then I’ll have learned something useful.


Consider me the peanut gallery, this is interesting to me.


So… from a rudimentary google search:

DPS Arms FAQs page: #8. Is there a difference between 5.56 NATO and .223 Rem? Yes, despite external dimensions being identical, 5.56 NATO ammunition is a higher pressure round than commercial .223. If your rifle is chambered in .223 only .223 should be used as using .556 NATO can create an unsafe high pressure condition. If your rifle is chambered in 5.56 NATO it is safe to fire both .223 and 5.56 NATO.

Although most of the rifles on their site are listed as .223 & 5.56 NATO, their bull barreled target AR is listed as being chambered in only .223: DPMS Sweet 16.

This follows my assumption from the YT video and NRA article I listed that the .223 only chamberings might have a potential accuracy advantage (leade differences).

Although I can’t vouch for the third party posts (therefore I won’t link them), if you search on other forums, there seems to be two different answers regarding Remington’s R-15 line. Some customers pasted a response from Remington that they are .223 Rem only, while others pasted Bushmaster’s response (who supposedly made the Remington R-15 barrels) saying that the chambering was a hybrid with the tighter 223 dimensions, but with a leade resembling that for the 5.56 to handle the increased pressure. Furthermore, it’s confusing because their website doesn’t have a 5.56 option on their rifle website pages.

Interestingly enough, when looking at bolt actions, CZ states that it uses CIP (instead of SAAMI), and that CIP uses the 5.56 pressures to proof both the .223 Rem and 5.56 rifles coming out of that side of the Atlantic.

Further adding to the confusion, the website Balistics Tools goes into the similarity of higher pressure 223 to 5.56, but states that “pressures in any gun are affected a great deal by the throat and leade geometry” and that “The worst case scenario would be a full power 5.56 load, with a long bullet, fired from a brand-new .223 chamber that was cut to tight dimensions, with very little freebore, fired on a hot day, from brass with a lot of neck tension. Such a tight chamber is common on accurized match bolt-action .223 rifles. In such worst case conditions, pressures above 70,000 PSI have been reported. This is close to the proof pressure for .223, so there is potential danger firing 5.56 from a gun with a .223 chamber.”

Side note: the 70,000 PSI in the above statement refers to CIP standards (different PSI measurement method), not SAAMI.

Therefore, in a semi automatic rifle (which can get hot quickly) I might err towards caution when considering firing 5.56 in a .223 Rem only chambered rifle.


Ok, allow me to point back to my OP.
My point was and remains, AR’S are made to fire 556 or 223, AND the internal dimensions of those AR’s are no different.
Also I was pointing out that the main difference between the two cartridges, pressures aside, is the thickness of the brass at the base of the round, which sure, will allow for some hotter loads, but is that way to avoid ripping and jamming in full auto fire.

We can speculate, or even cite, fine differences in the round all day long. But the average Joe/Gina walks into a firearms shop, buys an AR, and it will handle 556 or 223 just fine. They’re not in separate sections on displays for each caliber because it matters not, the rifles are the same internally.

Ummm no actually they are not. AR chambers are cut completely differently than bolt gun chambers. And match chambers on AR’s are cut differently than “run and gun” chambers. Where the lands and grooves start ( the leade ) makes a huge impact on pressure. A 223 chamber will have a much closer leade than will a 5.56.

I think I have only heard of very few AR platforms that are specifically cut for .223 and they are primarily precision varmint guns twisted with 1-9 to 1-12 tubes. So I agree that in general you could buy any AR off the shelf and stuff in any ammo you want as most are cut for 5.56 or .223 Wylde.

The big issue is more on the bolt gun side of the house where leade length, angle and twist rate are optimized for a specific bullet and speed combination. So the varmint guy gets his tube twisted and cut for a specific 55 gr boolet, set to a specific point off the lands or touching them. All well and cool until his buddy hand him a couple rounds of military “Green Tip” which is not only longer but heavier and he has to crank the bolt shut to get it to chamber. Let that rifle and cartridge bake in the Wyoming sun for a couple hours and then light it off and you could have a problem.




That’s an excellent analogy @CHRIS4 !!!


@Smiddy I checked my Hornady Reloading Handbook and noticed the maximum cartridge overall length (COL) for .223 Remington is 2.260" and the 5.56x45mm is 2.250". Hornady also mentions the higher pressure load for 5.56 (60,000 psi vs 55,000 psi). The chamber of the .223 is longer than the 5.56. The .223 is the original Armalite caliber.


You are totally right!
The pressures are different!

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